improve the addition of slack penalties

We should add the option for users to set slack penalties and also implement a way to define the slack adequately.

For instance we we can take the structure

mutable struct ObjectiveFunction
    invariant_terms::JuMP.AbstractJuMPScalar
    variant_terms::GAE
    synchronized::Bool
    sense::MOI.OptimizationSense
    function ObjectiveFunction(invariant_terms::JuMP.AbstractJuMPScalar,
        variant_terms::GAE,
        synchronized::Bool,
        sense::MOI.OptimizationSense = MOI.MIN_SENSE)
        new(invariant_terms, variant_terms, synchronized, sense)
    end
end

and add a slack variables vector that doesn't get added until the end when we call get_objective_expression that dynamically sets the penalties for the slacks.

Start by locating the ObjectiveFunction structure and the get_objective_expression entry point mentioned in the issue. Determine how slack variables and penalties should be represented, then confirm that users can define slack behavior and set penalties dynamically when the objective expression is created.